1. Field of the Invention
The present invention relates to electrode material to produce an electrode of a spark plug for internal combustion engines.
2. Description of the Related Art
In view of the recent trend of preventing global warming and is consumption of fossil fuel from increasing, the pollution control standards act and regulations to chemicals, particulate matter, or biological materials contained in exhaust gas emitted from internal combustion engines for motor vehicles become stricter year by year. In order to achieve this, because a combustion temperature of the internal combustion engine of a motor vehicle, etc. tends more to increase, a spark plug is more required to have a superior durability and long duration.
Various types of nickel base alloy (Ni base alloy) are widely used as an electrode material to be used for producing an electrode of a spark plug in view of oxidation resistance, spark wear resistance, high strength at elevated temperatures, etc. Recently, various types of spark plugs, other than the above spark plug having an electrode of Ni base alloy only, have been produced and used in order to be better protected against a high temperature environment. For example, a recent spark plug has an electrode having a double layer structure composed of a bottom part and a top part. The bottom part is made of Ni base alloy. The top part at which the spark is generated is made of noble metal in order to improve its lifetime. Another recent spark plug has an electrode made of Ni base alloy and a high conductive metal such as Ag and Cu as a core material of the electrode.
For example, it has been proposed that such a Ni base alloy is used as S the electrode material of a spark plug (for example, which is exposed to the inside of a combustion chamber of an internal combustion engine) contains approximately 3 mass % of Cr in view of obtaining easy workability, high oxidation resistance and high temperature strength.
It has also been proposed to use Ni base alloy which contains additive element in order to further improve the oxidation resistance. Specifically, the following conventional patent documents 1 to 5 have disclosed various electrodes of spark plugs in which one or more elements such, as Si, Mn and Al, or Y and rare earth elements are added into Ni base alloy. The Ni base alloy contains a low concentration of Cr which is added to obtain oxidation resistance.    Patent document 1: Japanese patent laid open publication No. S63-18033;    Patent document 2: Japanese patent laid open publication No. H02-34734;    Patent document 3: Japanese patent laid open publication H02-34735;    Patent document 4: Japanese patent laid open publication No. H04-45239; and    Patent document 5: Japanese patent laid open publication No. H09-235637.
Electrode material made of Ni base alloy which contains a relatively low concentration of Cr is an excellent material in view of workability. However, because alloy as the electrode material needs additive elements such as Si, Mn and Al (having an excellent oxidation resistance) instead of a decreased amount of Cr (having an effective oxidation resistance) and because a total amount of elements contained in the Ni base alloy is thereby increased, the electrode material of Ni base alloy tends to decrease the thermal conductivity and its melting point. That is, the more the amount of the additive elements is increased in order to increase the oxidation resistance capability, the more it is difficult to decrease the temperature of an electrode of the spark plug because of its low thermal conductivity, and the more it is easy to decrease the melting point of the electrode material forming an electrode of a spark plug and to cause spark wear of the electrode of the spark plug when the spark plug works.
By the recent trends of internal combustion engines to increase performance, combustion efficiency, and working load, the spark plug use-environment becomes further stricter. It becomes difficult for the conventional electrode material made of Ni base alloy containing a low concentration of Cr and additive elements to show sufficient characteristics.
Therefore, the inventors of this patent applied another conventional technique disclosed in patent document 6, Japanese patent laid open publication No. 2006-316344. The conventional technique disclosed in the patent document 6 provides a spark plug containing additives such as a small amount of Si, a decreased amount of Mn and Al, and a small amount of one or more kinds of rare earth elements or Y in order to have high thermal conductivity, high melting point, good oxidation resistance and high spark wear resistance.
However, there is still a demand or a room for improvement in view of oxidation resistance, workability and manufacturing cost of the electrode material to be used of producing an electrode of a spark plug disclosed in the conventional patent document 6.